This invention relates generally to lighting fixtures, and, more specifically, to highly efficient optical systems having multiple reflectors which are shaped and positioned to produce an even distribution of light over a broad illuminated area without glare.
The efficiency of lighting fixtures for outdoor illumination purposes is not merely a function of the number of lamp lumens emitted per unit of power consumption, but rather is a combination of several interrelated considerations. For example, an outdoor lighting fixture should illuminate as large an area as possible, and provide uniform lighting throughout the illuminated area. Further, visibility throughout the illuminated area should be maximized while eliminating unneeded glare. This can be accomplished, in part, by minimizing the amount of light thrown into the eyes of drivers or pedestrians, and by preventing the projection of light above the horizontal. Moreover, economic factors such as electric power consumption per illuminated square foot and the cost of lighting fixtures and poles, must be considered in determining the overall efficiency of a system.
It is especially important that rapid changes in the illumination levels over the lighted area be reduced or eliminated to enhance visibility. The effect of providing an even transition from brightly illuminated areas to less well lighted areas is to enhance seeing. Also, a lighting system should direct virtually all of the available light to the intended area of illumination, causing a fairly steep fall-off of illumination at the outer edge of the lighted area.
A number of lighting fixtures have been designed which attempt to satisfy this criteria by providing a plurality of reflectors which intercept light flux from a lamp and shape it into a cone of light which has its maximum candle power directed at a relatively high angle, taken from downward vertical, but less than 90 degrees. Nested reflectors have been used extensively to shape this light cone and to prevent any light from being projected upwardly, with the principle aim of uniformly illuminating a generally circular area. Such prior approaches have usually required the use of a multiplicity of reflectors and/or the use of reflectors having reflecting surfaces of different contours, resulting in light fixtures which are difficult and expensive to manufacture.
Although the size of the lighting fixture can be minimized when many reflectors are utilized, the required multiple reflections of light between the small spaces separating the reflectors reduces the lighting efficiency of the fixture, and results in a luminaire which is difficult to clean. On the other hand, if too few reflectors are utilized, either the luminaire becomes very large for a given performance or it is necessary for the light to be reflected back across the luminaire where the light source and/or its supporting elements interfere with the light and reduce the efficiency of the fixture. Thus, a builder of lighting fixtures is faced with a number of seemingly conflicting and irreconcilable design constraints.
Generally, lighting fixtures should have a very tightly controlled beam for best performance. Besides concentrating the intensity of the beam to provide a higher maximum candle power throughout a selected range, a tightly controlled beam can be more precisely directed at higher vertical angles than less tightly controlled beams to give the fixture a wider area of coverage without increasing glare. This feature results from tight beams having a more clearly defined cut-off angle, which permits their use in wide area illumination coverage in a manner reducing the amount of light directed at nearby drivers and pedestrians.
With point light sources, the beam width can theoretically be made as tight as desired. However, the high intensity discharge lamps used in many outdoor lighting fixtures are not point sources, but instead have light emitting arc tubes of varying lengths. When redirecting light beams by means of one or more reflectors, the tightness of the beam depends on the length of the arc tube and on the distance between the arc tube and the beam forming reflector. To obtain a tight beam with a relatively long arc tube, it is often necessary to provide a larger reflector situated at a greater distance than is desirable or acceptable. As a result, common practice has been to utilize a plurality of reflectors, and to reduce beam width by limiting the portion of the light emitted which is redirected by each reflector. But, as mentioned above, such a practice results in a less than ideal lighting fixture.
Accordingly, there has been a need for a novel lighting fixture which is capable of uniformly illuminating large areas in a highly efficient and economical manner. The improved fixture should utilize an optical system having a minimum number of reflectors for redirecting the emitted light, and be able to advantageously use unreflected and singly reflected light, as well as doubly reflected light, to form the illuminating beams. It would be preferable that both oppositely facing surfaces of each reflector be manufactured to have the same contour and curvature, and that the reflectors be spaced far enough apart to facilitate cleaning and maintenance. Finally, an aesthetically pleasing design which can form a tight beam and minimize glare would be highly desirable. The present invention fulfills these needs and provides other related advantages.